Design of BL40XU

The basic design concept of BL40XU is to use the fundamental undulator radiation as a quasi-monochromatic x-ray beam. This eliminates the use of a crystal monochromator which has a band-pass of the order of 10^-4, unnecessarily narrow in most experiments. The fundamental undulator radiation has an energy peak-width of 2 % and thus the flux is more than 100 times higher than that obtained with a crystal monochromator.

T. Hara, T. Tanaka, T. Seike, T. Bizen, X. Marechal, T. Kohda, K. Inoue, T. Oka, T. Suzuki, N. Yagi, and H., Kitamura. In-vacuum x-ray helical undulator for high flux beamline at SPring-8. (SRI2000 (Berlin) August 2000)Nucl. Instr. Meth. A467-468, 165-168 (2001).
K. Inoue, T. Oka, T. Suzuki, N. Yagi, K. Takeshita, S. Goto, and T. Ishikawa. Present Status of high flux beamline (BL40XU) at SPring-8. (SRI2000 (Berlin) August 2000) Nucl. Instrum. and Meth. A467-468, 674-677 (2001).

The x-ray source of BL40XU is a helical undulator. The core of its radiation has an energy spectrum with a very sharp fundamental peak with smaller peaks of higher harmonics (Figure 1). In fact, most of the power of higher harmonics is emitted off-axis. On the other hand, the energy of the fundamental radiation is concentrated in the core: even when only the central 15 micro-radian (horizontal) x 5 micro-radian (vertical) radiation is used, the flux is as high as 1.5 x 10^15 photons/sec.

The undulator gap can be varied so that the fundamental radiation is altered between 8 and 17 keV.

Figure 1

Front End
The elimination of higher harmonics helps reduce the heat load on the optics. The first optical component, which is a horizontally focusing mirror, receives only 7 watts of power. The rest of the power is absorbed by the water-cooled fixed slits in the front end. They are fixed at the center of radiation but they can be moved to scan the radiation when the center of radiation is searched during commissioning. The front end slits are located behind these fixed slits. They will be used with an aperture of less than 15 x 5 micro-radian in most experiments. However, the aperture can be opened up to 50 x 50 micro-radian (accurately, it is 1.0 mm at 22 m from the source) for experiments which require quasi-white (pink) radiation. Such a large aperture has to be used with caution because the heat load on the optics may cause instability of the x-ray beam.

The focusing optics consists of horizontally and vertically focusing mirrors which are made of silicon and coated with rhodium. Both mirrors are water-cooled. The glancing angle of the first (horizontally focusing) mirror is set to 3 mrad and the second mirror to 4 mrad to eliminate higher harmonics. The 700-mm long horizontal mirror is placed first because the beam is larger in width than height: at a fixed glancing angle, the footprint of the beam is larger on a horizontally focusing mirror giving lower heat density. The horizontally-focussing mirror is also coated by nickel, and there is also an uncoated region. The coating can be changed to adjust the energy cut-off.

Experimental Hutch
The mirrors are located about 4:1 position between the undulator source and the focus of the beam. Thus the beam size at the focus is about 1/4 of the source, although the surface unevenness of the mirrors may affect the beam size. For the actual beam size, see the commissioning report below.

The experimental hutch can accomodate most equipments. There is a table that has fast shutters: one is driven by a galvanometer-like motor and opens and closes within 1.5 msec after a trigger pulse. The other is a rotating-aperture type shutter. By synchronizing the two shutters, 10 micro-sec opening can be achieved. This table also has final slits.

A table of required size can be used in various experiments. Thus, nothing is fixedin the experimental hutch except the table for shutters. A vacuum pipe of 3 m can be set up for small-angle experiments. A fast CCD camera with a framing rate of 291 per sec (640 x 480 pixels, 10 bits) is installed with an x-ray image intensifier which has a short-decay phosphor. By reducing the size of the frame, a framing rate up to 5000 per sec is achievable.

A YAG laser is installed in the hutch for experiments that require quick trigger of events.

For single-bunch experiments, a Julich chopper is available.